Under and over the ice

I really like the fact that there is still so much to discover about important parts of the climate system. The Bell et al paper in Science Express this week (final version in Science) reporting on the surprising results from airborne ground-penetrating radar studies of the Antarctic Ice Sheet is a great example. The ice sheets themselves are the biggest challenge for climate modelling since we don’t have direct evidence of the many of the key processes that occur at the ice sheet base (for obvious reasons), nor even of what the topography or conditions are at the base itself. And of course, the future fate of the ice sheets and how they will dynamically respond to climate warming is hugely important for projections of sea level rise and polar hydrology. The fact that ice sheets will respond to warming is not in doubt (note the 4-6 m sea level rise during the last interglacial), but the speed at which that might happen is highly uncertain, though the other story this week shows it is ongoing.

The radar results (shown on the right) picked up on some really weird looking features that look to be related to pressure-related freezing of basal meltwater as it is pushed uphill by the weight of the ice sheet above. If that sounds odd, it is because it is.

How can water flow uphill in the first place? This is a function of the pressure gradients. If there is a lot of ice above a valley, but it tapers off towards a mountain range, the pressure on any liquid water at the bottom of the valley will be greater than the pressure up the side of the mountain. This will force water uphill. Incidentally, there are many sub-glacialgeomorphological features that show this effect in places affected by the LGM ice sheets.

The freezing point of water is also pressure dependent. With 3km of ice pressure above it, water freezes at about -2ºC (the change is -7.5*10-8 ºC/Pa). Water below 0ºC can therefore exist at the base of the ice sheet (and can also be seen emerging from under ice shelves). When the pressure forces this water upwards to lower pressure areas, this can promote instant freezing, and this seems to be the explanation for the structures seen in the radar.

The surprise is how large these structures are, one shown in the paper is 10’s of km long and 100s of meters thick – certainly large enough to be important in ice flow locally, though probably not at the continental scale.

However, at the continental scale, there is a new assessment of the net mass balance of Antarctica and Greenland. Rignot et al have updated results, including those from the GRACE gravity measurement satellite, to the end of 2010 and show that the downward trend in ice mass is continuing (stronger in Greenland than in Antarctica). The net rise in sea level associated with this decline is about 1.3 mm/yr, which will likely accelerate with further warming. Complementary analyses of the surface mass balance of Greenland (Tedesco et al, 2011) also show that 2010 was a record year for melt area extent.

This rate of melting is more than was figured into the tabulated IPCC AR4 estimates of sea level rise, and any further acceleration will obviously make the discrepancy worse. Indeed, even in the highest forcing A1F1 scenario, the IPCC calculated only a 0.3 mm/year contribution from the ice sheets averaged over the whole 21st Century! This was clearly a gross underestimate.

Extrapolating these melt rates forward to 2050, “the cumulative loss could raise sea level by 15 cm by 2050” for a total of 32 cm (adding in 8 cm from glacial ice caps and 9 cm from thermal expansion) – a number very close to the best estimate of Vermeer & Rahmstorf (2009), derived by linking the observed rate of sea level rise to the observed warming.

There is certainly more to learn about ice sheets, and more of a reason than ever to do that as fast as possible.

119 Responses to “Under and over the ice”

Thank you. This is very interesting. It’s only yesterday that I read some articles and blogs by deniers dealing with Antartica, which is, according to them, the biggest proof that climate science is wrong, because the ice is not thinning and is, in some places, even getting thicker. Of course, I do not believe this. I do not believe anything from these people. Best regards, Will

[Response: Yup. It’s easy to focus on a place where we have the least information, to try to declare such information wrong. Of course, we do have enough information: the rate at which Antarctica is losing mass has been increasing along with Greenland. The rate of increased loss is an additional 14 billion tons/year, each year. See e.g. panel b in Rignot’s figure (site linked in the article above, figure here).–eric]

How is this likely to affect the interpretation of ice cores, if at all

[Response: Not much – this kind of ice is very different to compacted snow (no air bubbles for instance), and the isotopic values will be much more homogeneous than the rest of the core. So if they found this kind of ice, they’d know it. It might impact where people might drill though. – gavin]

Actually, it doesn’t seem odd at all that water would flow uphill in these conditions. Makes sense to me, anyway.

But what I don’t understand (depths of ignorance, freely confessed – probably a stupid question) is this: are the lateral forces on the mountain range in equilibrium? That is: is the ice exerting a force through the mountains, in the direction of whichever side of them has the more mobile ice forms, or not? If so, is the heat generated by that non-trivial?

[Response: You are correct. A straight line in these figures (with negative mass balance) would be a constant loss of ice, the slope indicates that the loss is increasing over the last twenty years. – gavin]

In 2010, the sea level contribution of Greenland and Antarctica is 1.67mm/y
The contribution of ice caps and mountain glaciers is 1.25mm/y (402 gt/y in 2006 with 12 Gt/y^2 acceleration).
The sum is about 2.9mm/y in 2010 and 2.52mm/y in 2006.
The average is 2.7mm/y.

With regard to the question of #7 regarding the steric sea level change in 2010, another term to add into the mix might be interannual variations in land storage. Also, when calculating a term in a budget as a residual, it is important to propagate the errors through the calculation. There may be a bit of a range on the steric sea level change estimate in this case.

Hansen conjectures exponential increase, rather than quadratic, but over the short timescale of the data, exponential and quadratic are indistinguishable. So Hansen’s constant doubling time idea is not ruled out.

[Response: Any extrapolation based on fitting an arbitrary function is problematic and will inevitably diverge from reality sooner or later – I would much rather have an estimate based on physics. But the meantime, these extrapolations need to be treated rather carefully. – gavin]

In 1992 the sea level rising was about 3mm/y.
It’s the same that the 2010 rising.
In 1992 the “melt” rising was 0.5mm/yy, so the steric was 2.5mm/y.
In 2010 the melt rising is (selon Rignot et al) 2.9mm/y.
So the steric is 0.1mm/y (OK there are error bars …))

If this steric rising is decreasing this is because (in the middle term)the radiative budget TOA is also decreasing.(but perhaps the climatic variability could help us?)

This is very “inconvenient”, I think.

No?

And I don’t think that land storage variations can explain this.

But maybe Gavin could give a response?

“On ne sait jamais…” [Response: I think you’re ignoring the uncertainties on all these numbers. If the new ice dynamics numbers make other estimates necessarily smaller, or larger, but still within their original uncertainties, this means ‘no detectable change’. Try those numbers again, but include the uncertainties as given in the published literature, and *then* see if there is anything that requires an ‘explanation’.–eric]

Just a sligthly off-topic and technical question (I can’t get access yet to the article, must check in my lab but I have doubts) : which wavelength (and instrument) did they use ? I read on the profile a 2000m penetration, which is very impressive considering we are dealing with ice (our own small hand-held radars can barely get to 20 meters …)

(Recaptcha : from turequa. Lord Inglip knows where it comes from …)

[Response: It’s all about wavelength of course, when it comes to penetration. For the bit of ice radar work I’ve done, it’s 5 MHz frequencies. Bell is using higher frequency, but has a lot more power than our little home-built devices. These instruments are all custom made, in Bell’s case I would guess they are using instruments built by the U. Kansas engineering group (see their web site https://www.cresis.ku.edu. From the paper. “150 MHz ice penetrating radars with bandwidths of 15- 20 MHz produced high-resolution images of the internal structure of the East Antarctic ice sheet.”–eric]

Familiarity with its molten state makes it easy to forget that water is a mineral- the first of the oxides in Dana’s classic System of Mineralogy.

And like all minerals abundant enough to form rocks, it is encountered in a variety of textures, including dynamically generated ones, termed mylonites, where the motion of rocks at interfaces has milled their hard and soft crystalline components into pulp.

As fine division accelerates ion exchange, some clays originate this way, and subglacial grinding sometimes contaminates admixed ice with silicate and clay mineral particles fine enough to dispersion strengthen the frozen bulk, stiffening some layers and rendering interfacial flow complex- basal melting is not the only determinant of ice rheology.

The clear bulk ice is a product of re crystallization, and recrystallization can erase dynamic history much as retrograde metamorphism obscures metas*matism in ‘dry’ rocks.

I’m curious how their findings relate to the isostatic adjustment issue raised by Wu et al. 2010 (doi:10.1038/ngeo938), which led the latter to conclude that mass loss in Greenland and Antarctica was roughly half what other recent GRACE-based studies had found.

Re: #3
While not an expert, it’s safe to say that ice cores drilled for climate study most probably would not be drilled in such areas. The exception might be for surface accumulation studies and then the cores would be relatively shallow. (Accumulation rates are so low that short cores can record centuries of data.) Cores for climate records tend to be drilled in unexciting places — ice divides, slow moving ice, places with little or no liquid precipitation (not a problem in most of Antarctica).

The ice sheet is relatively flat at the surface, with respect to the size of the mountain ranges and their relative topography.

Pressure is a scaler (strictly speaking for liquids), so that along the mountain ridges and integrating downward, the normal forces pretty much cancel thenselves out on either side, resulting in little to no net lateral forces upon the mountain ranges, in total (assuming full contact).

Ice sheets, like water, will seek the path of least resistance. Their flow field will be predominantly perpendicular to the bottom contours/topography, somewhat similar to slowly varying open channel flows of water.

The major heat source at the bottom of an ice sheet is primarilarly from within the Earth itself (thus the submerged lakes seen there, subjected to very high pressures).

I used to be a Research Hydraulic Engineer, in a previous lifetime, with much of my work involving naval architecture of floating and submerged bodies, so, for example, a neutrally buoyant submarine has a zero net force (and moment) acting upon it, the same applies to freely floating bodies.

Having said that, the ice sheet is moving relatively slowly, there are friction forces, separations occur with the bedrock, so that, as one would expect, erosion of the bed will occur, as the ice sheet grinds ever so slowly away on the bedrock.

In closing, mass continuity is preserved at the bottom of an ice sheet, it moves that liquid (under very high pressure) from point A to point B (in this case if the slope is steep, the high pressure water moves upwards, because the ice sheet loses it’s direct contact pressure with the bedrock (also remember rho water > rho ice), the water pressure is greater then the ice sheet contact pressure, then refreezes, the cycle repeats, and you get “ice growth” at the bottom-sides). Nothing gained and nothing lost. Kind of like robbing Peter to pay Paul, if you know what I mean.

Little to no heat is generated from within the ice sheets themselves, they can be heated from the Earth’s interior, or they can be cooled from the Earth’s surface.

Finally, here’s a link to the Rignot paper (I’m still looking/waiting for the Science weekly e-publication of Bell’s paper though, it might better explain the physics than what I have assumed in my explanation above, but I think I’m fairly close to guessing the correct physics, YMMV);

[Response: Other terms will oscillate – thermal expansion, ground water extraction, mountain glacier, irrigation demands etc. Given the uncertainties in those, we’ll only get a credible closure on relatively long decadal timescales. – gavin]

I’m not sure where you get the statement “even in the highest forcing A1F1 scenario, the IPCC calculated only a 0.3 mm/year contribution from the ice sheets averaged over the whole 21st Century”.

From Table 10.7 of the AR4 WG1, it seems we need to add the average contributions for “Greenland Ice Sheet SMB”, “Antarctic Ice Sheet SMB” and “Scaled-up ice sheet discharge”. Over the ~105 years between 1980-1999 and 2090-2099, these contributions are:

I do find this radar penetration technology very interesting, it might help them find places where the fabled 1.2-1.5MYa ice core(s) could exist, that they all all would like to drill for on the EAIS (the one where they pick up the 100KYa to 41KYa transition sequence). Can’t remember that acronym now, but older ice cores (AFAIK) are expected to be near mountain ranges, which are further removed from the Earth’s internal heating (as seen in this paper).

Sea level rise will not be uniform. Some coastal areas will see much large increases than others. But, I have not seen a study that gives estimates of which areas will see higher than average increases and which lower around the globe. Does anyone know of such a study?

Heh – there are conflicts in the two papers you’ve cited. But that’s the best of it, isn’t it?

Bestmann’s(et al) temperature numbers are huge (“…giving flash temperatures of 274 °C (slow speed model) to 239 °C (high speed model).”). And oddly, higher for slower-moving ice. I had to wade through the numbers on that – I thought he’d accidentally reversed a sign or somesuch. And that was 2006 – five years ago. We’ve added so much since then.

Sidd, the time scale doesn’t seem to be too short for isostatic adjustment to matter; all these studies correct for it one way or the other, Wu found their correction made a big difference over as short a period as 2002–2008, and Rignot covers two decades.

I’m not sure how to compare Rignot’s and Wu’s results (or whether they are comparable), but by a naive calculation, Rignot too gets more than half the Greenland + Antarctic mass loss rate of Wu.

Since Rignot et al. find good agreement of the GRACE gravity method with the totally independent mass budget method, I reason that their findings disconfirm the novel isostatic adjustment corrections of Wu et al.

But I’m a total layman here, so please clue me in if I’m reading it wrong.

Yep, the papers don’t conflict, they describe different situations; and those are just two out of a large number of papers. This isn’t simple and isn’t all measured, some is theoretical. There are some experts reading here, pointers welcome to papers that would help the rest of us with yet another huge field of science to learn about.

It needs to be remembered that there are significant temperature gradients in the solid ice. Those temperature gradients are the only way the geothermal heat is transported to the surface. In liquid, there can be convection, but the density of water is highest at 4 C (at one atmosphere).

The strength of ice is a very strong (and non-linear) function of temperature, at the melting point it goes to zero.

The higher density of liquid water is the reason that ice sheets always collapse catastrophically. The pressure at a column of water is higher than at the bottom of a column of ice, and at the melting point the ice has essentially zero strength. This is what causes melt water on top to flow to the bottom.

When that water meets ice that is at a temperature below the melting point, the water freezes and deposits its heat of fusion. That warms the ice up to the melting point. Once the whole ice column is at the melting point, there is no temperature gradient for heat to flow down. The ice at the base stays at the melting point where it has very little strength.

My guess would be that there is a high temperature gradient up stream of the melt pocket and a lower temperature gradient down stream. My guess is that what is keeping the system stable is the cold ice being convected into and past the melt pocket faster than the melt water can flow “upstream” (because the cold ice is strong enough to resist the pressure differential of the water vs ice hydrostatic pressures).

At some point, as the temperature of the ice flowing down stream gets higher, it won’t be enough to maintain the ice cold enough to maintain its strength to resist the flow of water. Once the water flow starts, it will warm that ice and weaken it still more and there will be positive feedback until it collapses catastrophically.

As I recall, some of the earlier work on Greenland ice balance and ice sheet stability was done via altitude measurements and indicated that there wasn’t much change.

The GRACE measurements contradicted this and did show a reduction in total ice. My question, is the observation of not much change in the height of the top of the ice while there is a change in the amount of ice present an indication that the density of that ice is going down; in other words that the average temperature of the whole ice column is going up?

Dan H. above points to the recent open access article (full text available):
Climatic Change (2010) 100:733–756 DOI 10.1007/s10584-009-9689-9
(I don’t see any surprises there; it’s a new method proposed and evaluated for getting more detail out of the available proxies; nice graphics).

Re # 32 I concur strongly, and suggest that the Missoula floods (and similar) were the result of a progressive collapse of a valley full of ice at its melting point rather than the discharge of a melt water lake.

Thus, apparently solid ice can convert to a Jökulhlaup without a volcanic trigger.

> Are you saying that the temperature increase in
> Greenland is expected due to long term fluctuations?

Nonsense. I’m not saying that.
The paper you pointed to isn’t saying that.
A few denial sites are saying that. They’ve been saying “temperature increase … is expected due to long term fluctuations” regardless of the science. That’s a common PR denial talking point.

Read the paper you pointed to.

They describe a new technique that may get better information out of the available proxies and give better information about the natural variability in the past.

Anthropogenic climate change is added on top of the natural variability.
No surprise there.

Hank,
I read the paper. That is why I brought it up. The results are different from other proxies, not just a diffferent technique. What I am trying to discern is how much of the recent warming in Greenland (and glacier melt) can be attributed to climate change, and how much to natural variability.

If there’s no accelerating trend in sea level rise (cf. Topex-Jason) and if Rignot at al 2011 is correct, we whould conclude that the steric change (from thermal expansion) of SLR is decreasing.

But if so, where is the ‘missing heat’ (Trenberth) or ‘global warming still in the pipeline’ (Hansen) – heat storage in the ocean, whose first effect would be an increasing SLR from thermal expansion ? Does it mean that transient climate response (as expressed by ice sheet or see-ice melting among other events) to GHGs is not so far from equilibrium climate sensitivity ?

Post scriptum : furthermore, RC recently commented the Lyman 2010 paper, finding an increase in ocean heat content 0-700 m from 1993 to 2008, nearly the same period Rignot and Velicogna observe the ice-sheet increasing contribution. These causes should add for SLR, shouldn’t they ? Which if any of the three measurements (ocean heat, ice-sheet, SLR) is considered as the less robust in climate science community?

In light of the enormity of the current clamity in Japan, especially Sendai and the scale of the earthquake 8.9; taken with a whole cluster of similiary sizable and deadly quakes all within the past 3-5 years all over the globe how much study is being done on the correlation between global warming and tectonic activity?.
I have been hearing about large and destructive quakes almost on a monthly basis for a number of years now. To me it’s unusual that the last 10 years have been progressively the hottest on record (both land and sea) and it’s seems to fit with a corresponding increase in significant tectonic instability of the large continental plates and the smaller fault lines that crisscross them almost everywhere on the planet. I’ve asked this question before on this forum and I think Gavin or Eric said as far they were aware there was no connection. Have you looked into this any further perhaps? I’ve still got this annoying feeling in the pit of my stomach that the two are related but don’t know which direction to start.
Any feedback on this will be very welcome.
Thanks guys!

Have there been any studies examining if accelerating global ice melt rates have any impact on seismic activity?

With the immense volume of glacial ice being shed each year, we are looking at a tremendous reallocation of weight upon the earth’s surface in an exceptionally short time (by geologic standards). Considering that isostatic rebound occurs even where comparatively small glaciers have receded, is it not plausible that the billions of metric tons of ice melted annually is not only contributing to isostatic rebound, but affecting even the tensioned detente between tectonic plates? And that the consequent readjustments might be reflected as earthquakes?

I have been hearing about large and destructive quakes almost on a monthly basis for a number of years now.

Cell phone cameras and other lightweight, portable still and video devices mean that news agencies have a lot more footage to inundate us with. I suspect you’re noting more coverage rather than more earthquakes. I’m in my 50s and until recently you never got much footage of a post-quake tsunami, for instance. The last two large tsunamis, thailand a few years back and now japan, yielded more video than I remember seeing the rest of my life put together.

Thanks for the answer/link Didactylos. That’s a good point dhogaza regarding modern media saturation. And of course there are more seismographs in the world as well.

Accepting that there is no measured increase in earthquake frequency thus far, is it implausible to consider that at some point, the annual reallocation of billions of metric tons of ice, which has been suppressing the earth’s crust with its weight, would affect plate tectonics?

I am not fishing for catastrophes: there may be a scientific basis for this not to occur. I just wonder if there is a point at which the intensification/recalibration of the carbon cycle and hydrological cycle will correspond with an intensification/recalibration of the tectonic process.

I’m more than happy to agree with Didactylos(#45) and USGS on recent siesmic activity, and Dhogza’s (#46) point about the proliferation of cameras is also obviously correct. The worlds stock of large-scale tsunami footage has been massively boosted in the last decade. Some of the footage from Japan has been quite staggering, and also quite distressing.

Another point to add is that we should surely expect to see an increase in the human cost of earthquakes, tsunamis and vulcanism over time, due to the increase in global population.

Like Lawrence I have also posted questions about possible connections between climate change and seismic/volcanic activity. The response has generally been pretty negative on RC, but I think this is partly because in the trenches of the ‘Climate Wars’, there is little enthusiasm amongst climatologists to open another (somewhat speculative) front.

As far as i can see (and i am merely an interested layman who never even finished my enviromental science degree), amongst geoscientists there is considerably more interest in this link.

Everett(#44) might be interested in the results of a very quick google i did, which turns up this…

This article contains comments from 3 geoscientists who all refer to evidence of this link. All of the comments are actually from 2006, since which further evidence has been published.

When i previously quoted one of these geoscientists, Bill McGuire, in previous questions on RC, regular posters highlighted that McGuires research institute was partially funded by the insurance industry. This rather familiar ‘skeptical’ tactic of impuning a scientists professionalism was rather depressing.

He is the former director of the Benfield Hazard Research Centre at University College London, which is the largest academic hazard centre in Europe, and the author of over 300 books, articles and papers.

So i take pleasure in quoting him again, from the article linked to above…

Bill McGuire, professor of Geophysical Hazards at University College, spelled out the scenario further in a 2006 article in New Scientist, titled “Climate change: Tearing the Earth apart?”.

“It shouldn’t come as a surprise that the loading and unloading of the Earth’s crust by ice or water can trigger seismic and volcanic activity and even landslides. Dumping the weight of a kilometer-thick ice sheet onto a continent or removing a deep column of water from the ocean floor will inevitably affect the stresses and strains on the underlying rock,” he wrote. “[While] not every volcanic eruption and earthquake in the years to come will have a climate-change link… [As] the century progresses we should not be surprised by more geological disasters as a direct and indirect result of dramatic changes to our environment.”

McGuire has written an article about the Japanese disaster, published yesterday…